Dfb laser with a distributed reflector and photonic band gap

ABSTRACT

The invention relates to a semiconductor laser consisting of an active waveguide ( 3 ) comprising an active region surrounded by a filling material (5) and which is coupled to a distributed reflector ( 7, 8 ). Said distributed reflector ( 7, 8 ) is made from the aforementioned filling material ( 5 ) and is disposed along the length of the lateral sides of the active region ( 4 ) essentially parallel to same and in the form of a structuring having a photonic band gap along the longitudinal axis (X) of the laser. According to the invention, the structuring defines a first photonic crystal with columns (9) forming diffracting elements, said crystal comprising a mesh having dimensions of the order of the wavelength of photons in the guided mode which circulate in the active waveguide ( 3 ).

The invention relates to the field of optical devices, more specificallyto that of DFB (Distributed FeedBack) semiconductor lasers.

As is known by one skilled in the art, DFB lasers, such as for examplethose called “buried ribbon” lasers, generally include an activewaveguide which classically comprises an active region buried in afiller material, and coupled to structures which form a diffractiongrating (or distributed reflector) due to a periodic variation of theindex. In terms of the guiding structure, the active region can beincorporated into the heart of the guide, whereas the filler materialwhich surrounds it, with a smaller index, corresponds to its coating.

The distributed reflector causing a periodic modification of the indexof refraction, only the photons emitted by the active region and havinga wavelength dictated by the reflector contribute to the laseroscillation. Consequently a DFB laser theoretically exhibits a narrowspectral footprint which makes it a single-mode laser.

But, in practice, this is not exactly the case due to the complexity ofthe processes currently in use to produce distributed reflectors.Actually it is necessary for the reflections at the level of the ends ofthe laser (typically the opposing faces of the component) to be in phasewith the reflections induced by the grating of the distributedreflector. These phase conditions can only be obtained by positioningthe laser face in a very specific manner relative to the diffractiongrating, and with a precision significantly less than the wavelength.

Moreover, this manner of producing distributed reflectors allows thegrating strength to be varied only with difficulty, i.e. the couplingcoefficient between the optical wave and the grating, from one laser toanother on the same wafer. Similarly, it is difficult to vary thegrating strength with any precision along the longitudinal axis of thelaser.

Approaches have been advanced in an attempt to eliminate thesedifficulties. One approach simply consists in devoting one wafer to eachtype of distributed reflector. But this approach is costly. Anotherapproach is to disorientate the grating to reduce its grating strength.But this approach changes the Bragg wavelength of the grating, andmoreover photons can be diffracted in unwanted directions. It no longermakes it possible to vary the grating strength along the longitudinalaxis of the DFB laser.

Thus the object of the invention is to eliminate the aforementioneddifficulties in whole or in part.

To do this the invention proposes a semiconductor laser of the typedescribed in the introduction and according to which the distributedreflector is implemented in a filler material which surrounds the activeregion of the active waveguide, along at least one of the lateral sides(preferably two), and essentially parallel to the latter in the form ofat least one configuration with a photonic band gap along thelongitudinal axis of the laser.

Classically, it is considered that a laser structure extends in threedirection perpendicular to one another, one direction, which is called“longitudinal” (X), defining the longitudinal axis of the laser relativeto which its longitudinal extension L (or length) is defined, onedirection called “lateral” (Y) relative to which for example the lateralextension I (or width) of the active region of the waveguide is defined,and one direction called “vertical” (Z) in the direction of stacking ofthe layers and relative to which for example the vertical extension h(or height) of the active region of the waveguide is defined.Accordingly, the phrase “the distributed reflector is implemented alongat least one of the lateral sides of the active region” means that thereflector extends over the entire length L of the active region of thelaser, or over a portion of it, essentially parallel to the planedefined by the vertical (Z) and longitudinal (X) directions. Theexpressions “longitudinal extension”, “lateral extension” and “verticalextension” are used in preference to the more common terms “length”,“width” and “height” respectively because they are intended to designatethe geometrical characteristics of the structures and not simpledimensions.

Moreover, “photonic band gap configuration” is defined as an-dimensional grating (n being preferably equal to 2, but can likewisebe equal to 1 or to 3), with physical properties which allow control oflight propagation by precluding it for certain wavelengths in certainthree-dimensional directions.

The distributed reflector as claimed in the invention is thus used tofilter, in the longitudinal direction, certain wavelengths, and not toensure lateral and vertical confinement, which is ensured by the verycomposition of the buried active region. This makes it possibleespecially to fix the grating strength at any desired value by actingprimarily on the distance separating the grating from the active layer.

The first configuration extends preferably over the entire lateralextension (or width) of the filler material placed on either lateralside of the active region. But it could equally well extend over oneportion solely of this lateral extension.

Moreover, the first configuration extends over one portion of at leastthe extension of the active region following the vertical direction,preferably over its entirety, and over one portion or the entirety ofthe height of the filler material.

In one preferred embodiment, the first configuration is a first photoniccrystal formed by localized etching of the filler material in such amanner as to form hollow columns there or to leave columns of materialremaining there, these columns comprising a periodic grating ofdiffracting elements with a lattice in the horizontal plane, whichlattice has dimensions of roughly the wavelength of laser operation.Again the columns more preferably extend essentially parallel to thedirection of stacking of the layers.

In addition, the lattice of the grating of the first photonic crystalpreferably has the shape of a convex polygon, with a height which isselected as a function of the operating wavelength under study. Thisconvex polygon is preferably a regular polygon (i.e. with all sidesequal) such as a square, an equilateral triangle, or a diamond.

The first configuration is spaced away from the active region by adistance which is selected as a function of the grating strength whichis required for the distributed reflector, such that the photons with awavelength different from that of the photons in the mode under studyare extracted.

According to another feature of the invention, this distance can beeither constant or variable along the longitudinal extension (or length)L of the active region such that the grating strength can be variedlongitudinally. The interest in this first arrangement is that it allowslasers with improved spectral selectivity to be implemented, forexample, by controlling the longitudinal distribution of the powerdensity in the laser structure.

According to another feature of the invention, the filler material ofthe active waveguide can be provided likewise on at least one of thelongitudinal ends of the active region, in order to obtain, at adistance δL from the longitudinal ends of the first configuration,reflection means implemented in the form of a second photonic band gapconfiguration and extending essentially parallel to the lateralextension of the active region. This second configuration is used as therear mirror for the DFB laser. Moreover the distance δL can be chosen tobe essentially equal to a whole number times half the wavelength of thephotons in the guided mode (i.e. half the wavelength of laser operationin the filler material) such that the first and second configurationsdefine a Fabry-Perot type resonant cavity.

Preferably this second configuration extends over the entire lateralextension (or width) of the waveguide. But it could extend over thewidth of the active region and all or part of the lateral extension ofthe filler material which has been placed on either lateral side of theactive region (in the zones in which the first configuration is formed).Likewise, it extends preferably over the entire height (stackingdirection) of the active region, and more preferably again over theentire height of the filler material which surrounds especially theactive region.

Equally preferably the second configuration is a second photonic crystalformed by localized etching of the filler material in such a manner asto form hollow columns there or to leave columns of material remainingthere, these columns comprising a periodic grating of diffractingelements with a lattice in the horizontal plane, which lattice hasdimensions of roughly the wavelength of laser operation.

In this case, it is preferable that the columns extend essentiallyparallel to the vertical extension (or height h) of the active region,over all or part (at least the active region) of the waveguide. It islikewise advantageous for the lattice of the grating of the secondphotonic crystal to have the shape of a convex polygon, preferably aregular polygon.

The convex polygons of the first and second photonic crystals areadvantageously linked to essentially identical Bragg wavelengths.Moreover, in one preferred embodiment the first and second photoniccrystals are composed of convex polygons of different types.

Other characteristics and advantages of the invention will becomeapparent by reading the following detailed description and inspectingthe attached drawings, where:

FIG. 1 is a schematic cross section, in the plane (YZ), of a DFB laseras claimed in the invention, equipped with a first type of photoniccrystal,

FIG. 2 is a cross section along axis A of FIG. 1 (in plane (XY)),

FIG. 3 is a section identical to that from FIG. 2, but showing a DFBlaser equipped with a second type of photonic crystal,

FIG. 4 is a section identical to that from FIG. 2, but showing anotherDFB laser as claimed in the invention, equipped with a first photoniccrystal defining the distributed reflector, and with a second photoniccrystal defining the rear reflector.

The dimensions of the various components comprising the DFB lasers shownin the figures are not representative of their actual respectivedimensions.

The coordinate system (X, Y, Z) shown in FIGS. 1 to 4 defines theperpendicular directions in which the DFB laser structure of theinvention extends. The X axis defines the longitudinal direction of thelaser (along its length L). The Y axis defines the lateral direction ofthe laser (along its width l). And the Z axis defines the verticaldirection of stacking of the layers (by agreement, the “lower part” ofthe laser is the bottom of FIG. 1, and the “upper part” of the laser isthe top of FIG. 1).

Reference is made first of all to FIGS. 1 and 2 to describe a firstembodiment of a DFB laser as claimed in the invention.

This DFB laser comprises first of all a substrate 1 (for example of typen) made of a semiconductor III-V material such as for example InP orGaAs. It likewise comprises an active waveguide 3 formed by an etchingtechnique, located above the substrate 1 and composed of an activeregion 4 in which the photons of the guided mode are produced, andvertically surrounded by an undoped optical confinement material 12. TheDFB laser finally comprises a filler material 5, for example, p dopedInP, laterally and vertically surrounding the active waveguide 3. InFIG. 1 the boundary 2 between the etched region comprising the activewaveguide 3 and the region containing the filler material 5 is shown bythe dotted lines. This example of a DFB laser is of the type called“buried ribbon” due to its manner of implementation.

The active waveguide 3 is coupled to a distributed reflector made hereas two identical diffraction gratings 7, 8 which have been formed in thelateral parts of the filler material 5 which are located on eitherlongitudinal (or lateral) side of the active region 4, defined by theplane (X,Z) and essentially parallel to these sides.

Each grating 7, 8 is a specific configuration of the filler material 5,called a “photonic band gap” configuration. Accordingly in the followinga grating is incorporated into its configuration. This type ofn-dimensional grating (n being preferably equal to 2, but can likewisebe equal to 1 or to 3) is familiar to one skilled in the art. It isdescribed specifically in the article of E. Yablanovitch, J. Opt. Soc.Am. B 10, 283 (1993) “Photonic bandgap structures”. As its nameindicates, it is designed in such a way as to have one or more energybands which are forbidden to certain photons of light which are emittedin the active region 4. It thus makes it possible to control (filter)the propagation of light or even to prevent propagation of certain ofthese wavelengths.

As stated above, the gratings 7, 8 are thus used to filter certainwavelengths and not to ensure lateral and vertical confinement, which isensured by the very composition of the active region 4 and the fact thatit is surrounded by optical confinement material 12 and filler material5. But they have a complementary function. Their photonic band gap isactually selected to ensure “longitudinal” optical confinement for lightwith the desired wavelength of operation. This light must be able topenetrate into the grating laterally, but it “sees” the photonic bandgap in the longitudinal direction (X axis). In other words, thisforbidden energy band in some way prevents propagation of the light inthe longitudinal direction (X axis) in the grating, and according to itsphase relation to the faces, induces situations of resonance which makeit possible to choose the wavelength of the guided mode. It can likewiseexhibit this characteristic as inhibition of longitudinal propagation bycoupling between the grating and the wave emitted as a function of thereflection conditions of the faces.

Preferably, the configuration 7, 8 extends entirely over the length L ofthe active region 3. Likewise preferably the configuration 7, 8 extendsmore or less over the entirety of the width of the filler material 5which is located on either side of the active region 4. In fact, it isimportant that the configuration extends over a portion of the opticalmode. Preferably still the configuration 7, 8 extends entirely over theheight of the filler material 5 which is located on either longitudinal(or lateral) sides of the active region 4. As shown in FIG. 1, theconfiguration 7, 8 can likewise extend into a portion of at least thesubstrate 1.

In the example shown in FIG. 2, the configuration 7, 8 defines aphotonic crystal of holes in the form of columns 9 which extendessentially in the vertical direction (Z axis) and comprise diffractingelements which ensure periodic variation of the dielectric constant.These holes 9 can be formed using localized etching techniques such asdry etching and electron beam etching, quite familiar to one skilled inthe art. This implementation of the distributed reflector avoids stagesof regrowing which are often delicate and costly.

Alternatively, a photonic crystal composed of columns of material couldbe implemented. The columns are also formed by localized etching of thefiller material, but in such a manner as to allow the columns ofmaterial to remain there.

Viewed in a horizontal plane, the grating of the photonic crystal isperiodic and has a lattice in the shape of a convex polygon. The numberof periods and the spacing of the grating along the X axis (tied to thedimension of the lattice) and the dimensions of the holes (or columns)are chosen as a function of the wavelength of laser operation understudy. This spacing is typically on the order of this wavelength.Moreover, the type of grating (just like the spacing) is chosen as afunction of the desired photonic band gap.

As shown, this type is preferably a square. But it could also beequilateral triangles or diamonds which have sides with essentiallyequal sizes.

The photonic crystal, specifically due to its implementation, enjoys onemajor advantage. It in fact makes it possible to precisely control whatis called by one skilled in the art the grating strength or couplingcoefficient which characterizes the intensity of coupling between thegrating and the waves linked to the emitted photons. Due to the factthat the grating is located in the filler material, outside of theactive material, this grating strength in fact depends first andforemost on the distance d which separates the side of the grating 7, 8from the longitudinal side (or lateral side) of the active region 4 andwhich is a significant parameter with regard to filtration of the photonwavelength. This leads to the fact that adjustment of the coupling forcecan be obtained by simple dimensioning of the distance d in a manneressentially independent of the other parameters of the laser structure.

As shown in the embodiment from FIG. 2, the distance d can be constant.But, as shown in the embodiment from FIG. 3, the distance d can varyalong the longitudinal extension L (X axis) of the active region 4. Inthis version of FIG. 2, the photonic crystal 7, 8 exhibits a “left”portion located at a distance d1 from the active region 3, and a “right”portion located at a distance d2 from this same active region. In thisexample, the right and left portions have the same lattice shape(square), but they could have different shapes to make available at thesame time different photonic band gaps and different grating strengths.Generally the law of variation of this distance as a function of thelongitudinal position depends on the studied effect and can be obtainedusing simulation software available to optoelectronic componentdevelopment teams.

Typically the spectral selectivity of the laser can thus be optimized.This implementation allows for example spatial compensation which iscalled by one skilled in the art “hole burning”which results ininterference appearing in the grating due to the propagation of thefield in opposite directions. Actually when the laser structure isessentially uniform, the distribution of the power density generally hastroughs and loops. At high operating power of the laser, the carrierdensity in the active region is thus found saturated at the level of theloops, and this inhomogeneous distribution along the structure degradesthe spectral selectivity. The invention will henceforth make it possibleto easily control the power density distribution in the laser structure.

Another version can be likewise envisioned in which the distance d isconstant along the active region 4, but with a diffraction gratingcomposed of two or more photonic crystals, with different lattices tomake available locally different photonic band gaps.

The distance d or distances di (i=1, 2 in FIG. 3) are preferablycontrolled by masking during the lithography phase, ordinarily on thenanometer scale.

Reference is made to FIG. 4 to describe another embodiment of the DFBlaser as claimed in the invention.

This is a version of the laser shown in FIGS. 1 to 3, in which the rear(or left) longitudinal end of the active region 4 is likewise surroundedby filler material 5 in which a rear reflecting mirror 10 is formedwhich has been implemented in the form of a photonic band gapconfiguration, while the front longitudinal end, opposite, is aclassical mirror surface 11 of the component, a surface equipped with anantireflection coating. In one version, each surface of the longitudinalend could be equipped with a photonic band gap configuration.

This implementation is designed to reduce or suppress the variations ofspectral behavior and/or of the threshold and/or of the efficiencyand/or of other parameters which arise between DFB laser of the sametype due to variations of the magnitude of the wavelength fraction offace positions relative to the distributed reflectors.

The configuration of the rear mirror 10 is similar to that of thedistributed reflectors 7, 8 presented with reference to FIGS. 1 and 2.It could accordingly be implemented during the same stage. It extends inthe filler material essentially perpendicular to the plane (X, Y) andthus essentially in the vertical direction Z.

This configuration 10 (which is incorporated below into the rear mirror)extends over a portion at least of the width I of the waveguide 3, andat least over the entire width of the active region 4 and the zone oflateral extension of the first configuration 7, 8. Moreover, thisconfiguration extends at least over the height of the active region 4,but preferably over the entire height h of the waveguide 3.

In addition, as in the first configuration, this second configuration 10defines essentially a second photonic crystal of holes 13 which extendsessentially along the vertical direction (Z axis) and comprisesdiffracting elements which ensure periodic variation of the dielectricconstant. Alternatively, a photonic crystal of columns could beimplemented in place of the photonic crystal of holes.

Like the first photonic crystal, this second photonic crystal ispreferably a periodic grating and has a lattice in the form of a convexpolygon. Likewise, the number of periods and the spacing of the gratingalong the X axis (tied to the dimension of the lattice) and thedimensions of the holes (or columns) are chosen as a function of thewavelength of laser operation under study. This spacing is ordinarily ofthe magnitude of this wavelength. Moreover, the type of grating (justlike the spacing) is chosen as a function of the desired photonic bandgap.

As shown in FIG. 4, this type is preferably an equilateral trianglelattice. But it could equally well be squares (possibly identical tothose of the first photonic crystal) or diamonds with sides ofessentially equal size. In fact the lateral grating 7, 8 must have aband gap along the longitudinal axis, but preferably not along thelateral axis, while the grating 10 at the end of the laser canpreferably have a band gap in these two directions (lateral andlongitudinal).

Convex polygons of the first and second photonic crystals are preferablydimensioned such that they have essentially identical Bragg wavelengths.

The second configuration 10 is formed at a distance δL from the rearlongitudinal end of the first configuration 7, 8. This implementation ofthe reflector using the second photonic crystal allows precise control,ordinarily on the nanometer scale, of the distance δL which contributessignificantly to the properties of the DFB laser, and especially to itsrejection rate of secondary modes (“SMSR”), its threshold and itsefficiency.

Moreover, this distance δL can be chosen to be essentially equal to awhole number times half the wavelength of the photons in the guided mode(i.e. half the operating wavelength in the filler material) such thatthe first and second configurations define a Fabry-Perot type resonantcavity tuned to the wavelength of laser operation.

A DFB laser equipped with a distributed reflector with variable distancedi of the type shown in FIG. 3 can be likewise equipped with a rearmirror and/or a front mirror with a photonic band gap, of the type shownin FIG. 4.

Typical dimensions of the elements of a structure of the type shown inFIG. 4 are given below, by way of a nonrestrictive example.

In order for example to implement a set of laser chips covering thewavelengths of band C on the same board, a broadband end mirror can beformed comprising twenty rows of equilateral triangular crystals, withan air filling factor of roughly 0.3 and a period (distance between thecentroids of the triangles) of roughly 340 nm, for light incident in thedirection parallel to the mid-perpendicular of the triangles. The periodcan exhibit variations of roughly 10 nm, given that broadbandreflections of roughly 100 nm are expected.

The lateral reflector, with an air filling factor of roughly 0.3, can beimplemented using a single one-dimensional (1D) grating with modulationfollowing the X axis and a Bragg wavelength matched to the laseremission wavelength. To obtain an emission wavelength of roughly 1550nm, with a device which has an effective index of roughly 3.2, theperiod should be roughly 242.2 nm. The lateral extension of the crystalmust be at least equal to that of the transverse mode, or typically 10microns from each side.

The separation distance between the longitudinal (or lateral) sides ofthe active region 4 and the lateral reflectors placed on either of thesesides is on average roughly a micron. But it can vary locally from 0 to10 microns in order to cover a wide range of grating strengths.Moreover, for a tolerance of roughly 10 nm over the laser emissionwavelength, it is necessary to adjust the period to approximately ±0.8nm.

The optical distance between the lateral reflectors and the endreflector can be equal to roughly 242.2 nm in order to be roughly halfthe wavelength of laser operation in the filler material. Moreover, thesame tolerance as that accepted for the lateral reflectors can beenvisioned for this optical distance.

This structure makes it possible to operate with a wavelength of roughly1550 nm in air.

The invention is not limited to the above-described implementations,only by way of example, but it encompasses all versions which can beenvisioned by one skilled in the art within the framework of thefollowing claims.

1. Semiconductor laser comprising an active waveguide (3) extending inthe longitudinal (X), lateral (Y) and vertical (Z) directions,comprising an active region (4), surrounded by a filler material (5) andcoupled to a distributed reflector (7, 8), characterized in that saiddistributed reflector (7, 8) is implemented in said filler material (9)along at least one of the lateral sides of the active region (4) andessentially parallel to them, in the form of at least a firstconfiguration (7, 8) with a photonic band gap along said longitudinalaxis (X).
 2. Laser as claimed in claim 1, characterized in that saidfirst configuration (7, 8) extends over one portion at least of theextension (h) of the active region (4) in the vertical direction (Z),and over one portion at least of the extension (h) of the fillermaterial (5) in the vertical direction (Z).
 3. Laser as claimed in oneof claims 1 or 2, characterized in that said first configuration (7, 8)is a first photonic crystal formed by localized etching of the fillermaterial (5) in such a manner as to form hollow columns (9) there or toleave columns of material remaining there, these columns comprising aperiodic grating of diffracting elements with a lattice in thehorizontal plane, which lattice has dimensions of roughly the wavelengthof laser operation.
 4. Laser as claimed in claim 3, characterized inthat said columns (9) extend essentially parallel to said verticaldirection (Z) of the active region (4).
 5. Laser as claimed in one ofclaims 3 or 4, characterized in that said lattice of the grating of thefirst photonic crystal has the shape of a convex polygon,
 6. Laser asclaimed in claim 5, characterized in that said polygon is a regularpolygon.
 7. Laser as claimed in one of claims I to 6, characterized inthat said first configuration (7, 8) is spaced away from the lateralsides of the active region by an essentially constant distance (d). 8.Laser as claimed in one of claims 1 to 6, characterized in that saidfirst configuration (7, 8) is spaced away from the lateral sides of theactive region by a distance (d1, d2) which varies along the extension(L) of said active region (4) in the longitudinal direction (X). 9.Laser as claimed in one of claims 1 to 8, characterized in that saidactive waveguide comprises, on at least one of the longitudinal ends ofthe active region (4), a filler material (5) in which, at a distance 6Lfrom the first configuration (7, 8), reflection means (10) are formedwhich are implemented in the form of a second photonic band gapconfiguration and extending essentially parallel to the extension (1) ofthe active region (4) in the lateral direction (Y).
 10. Laser as claimedin claim 9, characterized in that said second configuration (10) extendsat least over the entire extension (h) of the active region (4) in thevertical direction (Z).
 11. Laser as claimed in one of claims 9 or 10,characterized in that said second configuration (10) extends over theentire extension (1) of the active region (4) in the lateral direction(Y), and over one portion at least of the extension of the fillermaterial (5) in the lateral direction (Y).
 12. Laser as claimed in oneof claims 9 to 11, characterized in that said second configuration (10)is a second photonic crystal formed by localized etching of the fillermaterial (5) in such a manner as to form hollow columns (13) there or toleave columns of material remaining there, these columns comprising aperiodic grating of diffracting elements with a lattice in thehorizontal plane, which lattice has dimensions of roughly the wavelengthof laser operation.
 13. Laser as claimed in claim 12, characterized inthat said columns (13) extend essentially parallel to said verticaldirection (Z) of the active region (4).
 14. Laser as claimed in one ofclaims 12 or 13, characterized in that said lattice of the grating ofthe second first photonic crystal has the shape of a convex polygon. 15.Laser as claimed in claim 14, characterized in that said polygon is aregular polygon.
 16. Laser as claimed in one of claims 9 to 15,characterized in that said distance δL is essentially equal to a wholenumber times half the wavelength of laser operation in the fillermaterial such that the first and second configurations (7, 8; 10) definea Fabry-Perot type resonant cavity.